An entity which is physically non-existent; the hook is not something you want to be on. The hook is that which if someone or something is off of it, that particular thing is of a highly reputable, A.K.A., cool status. The hook is something similar to "the chain", which is also a non-physical thing which one desires to be off of rather than on.

The shit is off the hook gangsta.
("the hook" may be replaced with "the chain")